This study was undertaken to determine whether marked left ventricular hypertrophy may predispose to the occurrence of ventricular tachycardia in patients with hypertrophic cardiomyopathy. Extent of left ventricular hypertrophy was assessed, using two-dimensional echocardiography, in a group of 30 patients with hypertrophic cardiomyopathy in whom ventricular tachycardia had been documented on 24-hour ambulatory ECG monitoring, and compared to 61 patients with hypertrophic cardiomyopathy who had normal ambulatory electrocardiograms. Severe hypertrophy, involving at least three of the four segments in which the left ventricle had been divided, was significantly more common in patients with documented ventricular tachycardia (16 of 30, 53%) than in those with normal ambulatory ECGs (13 of 61, 21%; p less than 0.002). Conversely, mild hypertrophy, involving only one left ventricular segment, was significantly less common in patients with ventricular tachycardia (5 of 30, 17%) than in controls (32 of 61, 52%; p less than 0.001). Moderate hypertrophy, involving two of the four left ventricular segments, occurred about as frequently in patients with ventricular tachycardia (9 of 30, 30%) as in patients with normal ambulatory ECGs (16 of 61, 26%; p less than 0.05). In addition, the left ventricular hypertrophy, was also significantly higher (thereby indicating a greater magnitude of hypertrophy) in patients with documented ventricular tachycardia (72 plus/minus 17 mm) than in those with normal ambulatory ECG recordings (61 plus/minus 14 mm; p less than 0.005). In conclusion, our data show a strong association between magnitude of left ventricular hypertrophy and occurrence of ventricular tachycardia in patients with hypertrophic cardiomyopathy; these findings provide new insights into the components of the disease process associated with ventricular tachycardia.